Several devices corresponding to the above general definition have already been proposed. They permit the obtaining of an image of the course at the moment when the competitors cross a determined line, in particular the finish line. The image takes account moreover of the photographic identity of each of the runners and of the rank which they occupy relative to one another thanks to a time scale which is recorded on the same image.
One of the oldest of such arrangements generally called photofinish, is known from the Swiss Pat. No. 399.028 (=GB No. 1.045.434). It includes an apparatus comprising a photographic camera the film of which advances with uniform motion, means arranged to photograph only a narrow space or one line at a time, and means enabling the projection of time indications to be recorded on the film.
This apparatus is illustrated on FIG. 1 of this description. The moving body is displaced at a speed Vm in front of the lens 1 of the camera. Behind the lens is to be found a screen 4 provided with a slot 2. Behind the slot runs a photographic film 3. The image of the moving body moves at the speed Vi in front of the screen while the film runs at the speed V. An adjustment is made so that the speed V is as close as possible to the speed Vi. The effect of the slot is that the film receives only a part of the entire image seen by the lens, this part being one line of the image. The running of the film behind the slot has as effect to expose the film successively in time with the same line of the image. The photofinish thus records a bidimensional image which represents according to an axis (perpendicular to FIG. 1), the profile of the intensity of a line of the image and in accordance with the other axis (in the plane of FIG. 1), the time evolution of this profile. The visual analysis of the image enables reconstituting the order in time of the appearance of different moving bodies in the field of the line. Finally, by the addition onto the film of reference points giving information on the exact time of recording, the appearance of the different moving bodies may be measured in time. This arrangement is employed for instance to measure the order and the time of passage of competitors over a finish line.
In the system which has just been described, the exposure time Te of film 3 is the time of transit of this film behind slot 2, i.e. Te=.delta./V, if .delta. is the width of the slot. In practice, there is desired an exposure time of the film which is as great as possible, so as to enable taking views of subjects poorly illuminated or passing at high speed. This exposure time is at the same time limited by the blurring of the image materialized for instance by trails appearing at the edge of the moving body and which are provoked by a difference of speed between the speed V of the film and the speed Vi of the image of the moving body. This blurring determines the time resolution of measurement.
The phenomenon of blurring may be explained by means of FIG. 2 which reproduces at an enlarged scale a part of FIG. 1 including screen 4 and slot 2. In this figure there has been represented the same point P of the image of the moving body which is displaced from A to B at speed Vi. During the same time lapse, the film is driven at a speed V slightly less than Vi in a manner such that when the image has advanced from A to B the film advances only from A to C. There results therefrom a trail from the point P which will leave an impression on the film over a length .DELTA.x which is a representation of the spatial blurring. To this spatial blurring there naturally corresponds a temporal blurring Tf which may be put into relationship with the exposure time Te of the film and which takes into account the spread between the speeds V and Vi. This relationship may be given as follows: ##EQU1##
In case speeds V and Vi are substantially equal, the factor .mu. approaches zero and it follows that Tf&lt;&lt;Te, which signifies that the exposure time is much greater than the temporal blurring.
Here there may be cited a practical example drawn from an athletic course. The film of the camera is adjusted to a speed V of 35 mm/sec. The image of the course moves past in reality at the speed Vi of 28 mm/sec, giving a factor .mu. of 0.2. If one specifies a temporal blurring Tf of 1 ms, there will result therefrom an exposure time Te of 5 ms according to the relationship (1) given hereinabove.
There results from what has just been said that the photofinish as known today is advantageous in view of the comfortable time of exposure which it provides. At the same time, it exhibits certain disadvantages. It requires in effect a chemical development of the film which delays access to the results of the course. It requires good accessibility to the camera which limits its utilization to certain positions. There may also be mentioned its relatively great space requirement and mediocre reliability in view of the mechanical parts making up its construction.
To overcome these difficulties, the assignee of the present invention has disclosed in Swiss Pat. No. 590.518 another system for determining the time separating the passages of moving bodies across a reference line substantially perpendicular to their trajectory. This system consists of employing a television camera which is directed onto the reference line, recording the signals provided by said camera and simultaneously signals furnished by time keeping means, and reproducing these signals by the aid of a monitor. In order to realize this, there is employed a camera by means of which a linear unidirectional scan is effected coinciding with the reference line, and a pickup effecting a linear bidirectional scan in a manner such that the successive scans of the reference line by the camera are spread out over the screen of the pick-up in a direction perpendicular to that in which is effected the unidirectional scans, that is to say in the direction of the trajectory of the moving bodies. This patent mentions that such camera may be of a conventional type, but wherein however the two scans have been permuted in a manner such that the more rapid scan is effected according to the vertical direction, the slower scan having been suppressed. As a variant it is mentioned that preferably a camera will be employed using diodes of the solid state type.
This system is shown on FIGS. 3 and 4 of the present description. Here the moving body is displaced at the speed Vm in front of the lens 1 of the camera. Behind the lens there is placed a unidimensional photosensitive device 5 using solid state sensors. The image of the moving body runs at speed Vi in front of the device 5. FIG. 4 shows how the device is built such including a row of photosensitive unit sensors 6 disposed side by side. The incident light coming from the reference line to be sensed produces charges on each of these sensors which charges represent the intensity profile of the line of the image at a given instant. These charges are periodically transferred in the sense of arrows 9 into a shift register 7 bearing as many elements 8 as the sensor 6. A clock signal 12 operating at high frequency transfers the contents of the line towards amplifier 10 in the form of a video signal 11. These video signals are then memorized, and subsequently visualized in a manner to represent in the form of an image the temporal evolution of the line under observation (finish line for instance).
Several uni-dimensional devices are presently available on the market. They are all equipped with a substantial number of sensors (&gt;1000) to assure high resolution. For further details on this subject, reference may be had to the technical notes of the manufacturers, for instance to the note concerning the device TH 7803 of Thomson-CSF. One may even obtain complete cameras from the Fairchild company under the reference CCD 1100C to 1500C.
In the system which has just been described, it has been seen that the incident light produces charges on the row of sensors 6 which charges are periodically transferred towards the output 10 via the shift register 7. The selection of the duration of this period which will be referred to as To, determines at the same time that of the exposure time Te and of the temporal blurring Tf which, since here the sensor is stationary, is equal to: EQU Tf=Te=To (2)
Thus, to repeat the same example suggested with reference to the photofinish, and if here one determines the temporal blurring Tf to be 1 ms, the exposure time Te is equal to 1 ms. It is understood that the unidimensional system is less favourable than the photofinish since it presents a relationship Tf/Te equal to 1, while the relationship was equal to .mu. in the system employing the photofinish.
There results from what has just been said that the unidimensional system as discussed above exhibits low luminous sensitivity which limits its range of application to scenes which have a substantial illumination.